When is a pixel not a pixel? When it's three.

There's
been a lot of debate on our discussion forums about the use of the "x
3" notation after the pixel count when describing Foveon's X3 sensors.
After a meeting with Foveon at PMA they made clear that they feel that
they conform to published JCIA (Japan Camera Industry Association) guidelines
which say "For DSCs that use a plurality of image sensors, the number
of image sensors and the number of effective pixels corresponding to each
image sensor shall be noted." Obviously this standard was published
before Foveon announced their new technology but the use of "x 3"
does seems logical enough.

Phil: As I expected this news article has stirred
up a whole lot of further debate. The confusion here comes about from
existing manufacturers use of the word 'pixel'. Our understanding of a
pixel is a single dot in the final output image, however we all know that
for a mosaic sensor that single output pixel has been made up from several
input photosites. Thus although we call an exisiting CCD '3 million pixels'
it (I suppose) be called '3 million photosites'. In which case the X3
sensor used in the SD9 would be called '10.3 million photosites'.

When debating this it would probably make more
sense to refer to the pixel as what we see in the output image and the
photosite as a single captured colour value from the sensor (input).

Warning: the text
below is NOT the entire standard, it is simply a reproduction of the section
relating to "Number of Effective, Total, Recorded and Output pixels".
For the entire
standard click here - PDF.

5 . Items to be noted in catalogs

Rules to follow in catalog descriptions are defined below for each item.

(1) Number of pixels

Due to technical innovations and the variety of DSCs, the 1998 Guideline,
which required DSC
vendors to note the specifications of image sensors on printed matter
and related software for
DSCs, now does not always seem to be realistic.

Therefore, this revised Guideline includes rules to indicate the number
of pixels in a way that
customers can understand the notations correctly and quickly, as instructed
below;

i)

"Number of Effective Pixels" should be listed
using the highest priority notation when the image capture performance
is described.

ii)

Therefore, in cases where the number of pixels other
than the "Number of Effective Pixels" is listed, such as
the "Number of Total Pixels" or the "Number of Recorded
pixels", DSC vendors should give careful consideration to ensure
that these are not confused with the "Number of Effective Pixels".

iii)

In cases where the number of pixels other than "Number
of Effective Pixels" are noted along with the "Number of
Effective Pixels", the "Number of Effective Pixels"
should be noted with the highest priority.

(1 - a) "Number of Effective Pixels"

The number of pixels on the image sensor which receive input light through
the optical lens, and which are effectively reflected in the final output
data of the still image.

The Number of Ring Pixels can be included in the "Number of Effective
Pixels" as a specification of the DSC.

Note

Ring Pixel

The reasonable, minimum number of pixels surrounding
the image area which are necessary for the filtering process.

Image Area

The surface area of an image sensor which reflects the
focal plane area represented by the output image.

Rules for notation

a)

The "Number of Effective Pixels" defined in
this Guideline shall be used whenever DSC
specifications are presented. The "Number of Effective Pixels"
defined in the specification of the image sensor itself shall not
be used, because the latter has a different definition from that defined
here.

b)

For DSCs that use a plurality of image sensors, the
number of image sensors and the number of effective pixels corresponding
to each image sensor shall be noted. If the total number of spatially
sampled pixels is also noted, the description should clearly state
that the noted number is the total number of effective pixels corresponding
to all of the image sensors, to avoid consumer misunderstanding.

c)

For DSCs that use an image sensor ( including a linear
sensor) that capture an image by
spatial or optical movement, the number of effective pixels of the
image sensor and thenumber of sampling positions should be noted.
If the total number of sampled pixels (ring pixels can be included)
is also noted, the
description should clearly state that the noted number is the total
number of effective pixels, to avoid consumer misunderstanding.

The "Number of Effective Pixels" including
ring pixels provides a somewhat bigger value
than the maximum number of recorded pixels, even for a camera having
no pixel interpolation.

b)

The pixels to be used for vibration compensation to
take motion pictures shall not be
included.

c)

The Optical black area shall not be included.

(1 - b) "Number of Total Pixels"

Specification for the image sensor which indicates the number of total
pixels on the image
sensor.

Rules for notation

a)

The "Number of Total Pixels" defined in this
Guideline shall only be used for explanations of the image sensor.
The "Number of Effective Pixels" defined in this Guideline
shall be used when specifications of the DSC are reported.

For DSCs that use a plurality of image sensors, the
number of image sensors shall be noted. Reporting only the total number
of pixels, instead of reporting the number of pixels in each image
sensor, shall not be allowed.

d)

All numerical values can be rounded off to two decimal
places.

Examples of notation

a)

"2 M Effective Pixels ( Using Number of Total Pixels
of 2.1 M CCD )"

b)

"Number of Effective Pixels 340k x 3 ( Using 3
x Number of Total Pixels of 380k CCD )"

(1 -c) "Number of Recorded Pixels"

The number of pixels comprising one picture frame which is recorded on
the digital recording
media.

Rules for notation

a)

The "Number of Recorded Pixels", or other
wording which has the same meaning, shall be used. The term "Resolution"
shall not be used for the number of recorded pixels

b)

The number of horizontal and vertical pixels of the
brightness signal shall be reported.
A notation using the order of horizontal number of pixels followed
by vertical number of
pixels is recommended, unless the DSC makes no distinction between
horizontal and
vertical image dimensions.

c)

An additional notation describing the image format,
either using a symbolic description
such as RGB or YCbCr, or using words such as "YC System",
"Line sequential system", or "Color difference system",
is recommended.
A notation such as "Monochrome signal", to indicate lack
of a color signal, shall be included when no color signal is recorded.

d)

Further notation of the composition ratio of the color
signal, such as "4:4:4" or "4:2:2", is recommended.

e)

Even in cases where the virtual number of pixels is
increased by image processing such as interpolation, the total number
of recorded pixels is regarded as the"Number of Recorded Pixels".

f)

All of the numerical values can be rounded off to two
decimal places.

Examples of notation

a)

"Number of Recorded Pixels 640 x 480 ( RGB 4:4:4
)"

b)

"Number of Recorded Pixels 1,280 x 960 (1.2M)"

c)

"Image format : YCbCr 4:2:2"

(1 -d) "Number of Output Pixels"

Number of pixels comprising one picture frame which is provided by the
DSC through the output transmission means.

Rules for notation

a)

Wording which indicates the number of pixels provided
by the DSC through the
transmission means, such as the "Number of Output Pixels",
the "Number of pixels
Communicated", or the "Number of Transmitted Pixels"
shall be used.

b)

The number of horizontal and vertical pixels of the
brightness signal provided by the DSC shall be reported.
A notation using the order of horizontal number of pixels followed
by vertical number of
pixels is recommended, unless the DSC makes no distinction between
horizontal and
vertical image dimensions.

c)

Additional notation of the image format is recommended,
but when the image format is described as part of the previously mentioned
"Number of Recorded Pixels", this additional notation can
be eliminated.

d)

Further notation of the composition ratio of the color
signal, such as "4:4:4" or "4:2:2", is recommended,
but when the composition ratio of color signal is described as part
of the previously mentioned "Number of Recorded Pixels",
this additional notation can be
eliminated.

e)

Even in cases where the virtual number of pixels is
increased by image processing such as interpolation, the total number
of output pixels is regarded as the "Number of Recorded Pixels".
A notation for "Compression" shall be included in cases
where the total number of output pixels is decreased as a result of
signal compression.

f)

All of the numerical values can be rounded off to two
decimal places.

Examples of notation

a)

"Number of Output Pixels 640 x 480 ( RGB 4:4:4
)"

b)

"Number of Transmitted Pixels 1,280 ´ 960
(1.2M)"

Note: "Number of Output Pixels" should be distinguished
from "Number of Recorded Pixels"